RFID tags are small sized devices, typically in a label format, that can be applied to or incorporated into a product, device or even animal for the purpose of identification and tracking of the item in question using radio waves. Some RFID tags can be read from several meters away and beyond the line of sight of the reader. These capabilities make the use of RFID tags very interesting over optical bar codes in product logistics, even if the data contained in the RFID tags would be equal to the UPC (Universal Product Code), EAN (European Article Number) codes traditionally used in bar codes. EPC (Electronic Product Code) codes used globally in RFID tags make it possible to store more information in a standardized manner to the RFID tags than has been possible in case of basic optical bar codes. Thus, RFID tags are becoming increasingly popular in everyday product logistics in many commercial fields.
RFID is currently also used in Electronic Article Surveillance (EAS) systems. In such an application, RFID technology is used to detect the presence or absence of an object within the reading distance of a reading device. Articles in these RFID based EAS systems are detected by detecting an RFID tag or RFID inlay that is attached to an article or the package of an article.
An RFID tag typically comprises an RFID inlay and an overlay structure forming the RFID tag. The RFID inlay is an electrically fully functional RFID transponder device, that is, a device that works as a transmitter and responder. The main components of the transponder are an electronic integrated circuit and an antenna. An inlay further comprises a substrate and other optional layers to support the transponder. The overlay structure of an RFID tag forms further mechanical support for the inlay and it can be used for printing trademarks, brand names etc. Overlays can be e.g. laminated or molded on the inlay. A typical RFID inlay is flexible, and, depending on the overlay, the RFID tag can be flexible or rigid. RFID inlays are typically sold in reels or rolls comprising hundreds to thousands of inlays. Generally the RFID tags can be either active or passive depending on whether they include an internal energy source, or they are operated with the electro-magnetic field generated by the RFID reader device.
According to “A BASIC INTRODUCTION TO RFID TECHNOLOGY AND ITS USE IN THE SUPPLY CHAIN” (LARAN RFID, January 2004) RFID tags can operate on several frequencies. Four frequency ranges are generally defined as: (1) low frequency (LF); frequencies below 135 kHz, (2) high frequency (HF); frequencies around 13.56 MHz, (3) ultra high frequency (UHF); frequencies between 860 kHz and 960 kHz, and (4) microwave; frequencies around 2.54 MHz. RFID tags can be designed to operate near the reader device, or far from the reader device. In case tags are designed to work near the reader device, the tags are known as near field tags, and the energy transfer from the reader device to the RFID tag is mostly through the magnetic field generated by the RFID reader. Data transfer from the tag to the reader device in near field case is enabled by inductive coupling, where the RFID tag changes its impedance, and the alternating load is detected by the reader device. Sometimes the communication in the near field is known as near field communication (NFC). In case the tags are designed to work far away from the reader device, the tags are known as far field tags, and the energy transfer from the reader device to the tag is mainly through the electric field. Part of the RFID tag operates as an antenna, and the RFID device gets its energy from the electric field. In the far field case, data transfer from the tag to the reader device is enabled by field backscattering. The theoretical limit between the near field and the far field is proportional to λ/2π, where λ is the wavelength of the electromagnetic radiation generated by the reader device, equaling to c/f, where c is the speed of radiation (i.e. light) and f is the frequency. As a result, the limit between near and far fields for a HF RFID system would be 3.5 m and for an UHF RFID system the limit would be 5 cm. One can also define a transition zone between the near field and the far field.
In the near field tags, the strength of the inductive coupling between the RFID tag and the RFID reader is proportional to the area enclosed by the wiring of the RFID inlay. In the far field tags, the wiring of the RFID inlay performs as an antenna, and the length of the wiring must therefore be proportional to the wavelength λ.
Even if these wirings can be made to meander in the inlay, these physical principles determine size limits for the RFID inlays, e.g. the minimum size.
Typically RFID tags (or in some cases the RFID inlays) are attached to the articles or packages thereof. In case the article or package thereof is small in size, the RFID tag can take up a large portion of the article or package. Many of such articles are sold in stores, where they are placed for sale to a hanger. The mounting hole for the hanger also takes up some space of the package already with limited space available. Therefore, especially in case of small articles, it may be difficult to find space both for the mounting hole for the hanger and the RFID tag.
The hangers and respective hanger holes used in the art have been standardized for their dimensions. The frequently used versions include, for example, the so called “Euroslot”, “Inverted T”, “Osha” and “Oval” which will be described in more detail later.